Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431	Biochemistry	Fall 2001
Lecture Notes:: 30 November	© R. Paselk 2001
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MITOCHONDRIAL TRANSPORT/COMMUNICATION, cont.

Last time we left off with ATP/ADP exchange. Today we'll look at exchanging reducing equivalents.

Reducing Equivalent Shuttles: In aerobic metabolism NADH from glycolysis must be regenerated to NAD⁺ in the mitochondria. Two shuttles are important in different tissues/organisms for this process:

• Glycerol Phosphate Shuttle: This shuttle is found in insect flight muscles, which sustain very high rates of aerobic glycolysis, and fast twitch muscle in humans. (Figure 21.33, G&G p 703). [overhead 18.23, P] The NAD⁺ is actually regenerated in the cytosol by the Glycerol-3-P DH reaction, which reduces dihydroxyacetone-P to glycerol-3-P:

The glycerol-3-P is then reoxidized to dihydroxyacetone-P by Flavoprotein dehydrogenase. This enzyme is situated on the inner mitochondrial membranes outer surface. It uses FAD to oxidize the glycerol-3-P to dihydroxyacetone-P, passing the electrons to CoQ (note the similarity to succinate DH, except for the location on the outer instead of the inner surface of the membrane). The organism can thus get 1.5 ATP equivalents for this NADH.

• Malate-aspartate shuttle: This is the common shuttle in mammalian systems. It is both more complex and more efficient than the glycerol phosphate shuttle, yielding 2.5 ATP/NADH. It can be thought of as occurring in two phases: 1) transport of electrons from the cytosol to the mitosol, and 2) regeneration of the cytosolic oxaloacetate (Figure 21.34, G&G p 703). [overhead 18.21, P] In phase 1 oxaloacetate in the cytosol oxidizes NADH to NAD⁺ using malate DH. Malate is then transported across the inner membrane in exchange for an a-ketoglutarate by Dicarboxylate translocase. Malate is then oxidized back to oxaloacetate in the matrix by Kreb's Cycle malate DH to give NADH in the matrix. In phase 2 the oxaloacetate is transaminated to aspartate, taking a-ketoglutarate to glutamate, which are transported, and the process balanced as in the figure in Horton.

Regulation of Mitochondrial Oxidative Phosphorylation

The ETS appears to be regulated largely by the availability of ADP and NADH. For most catabolic situations [ADP] will be the controlling factor. Note that the effects of [ADP] will integrate the regulation of TCA and Glycolysis with that of ETS.

PHOTOSYNTHESIS

Review chloroplast structure. Note how the thylacoids are stacked, with granal lamellae between stacked thylacoids, and stromal lamellae facing the plastisol (chloroplast matrix). The components of the photosynthetic ETS will be differentially distributed between these two lamellae. Most of protons will be pumped into the lumen of the thylacoid system. (The important thing here is that the protons are being pumped out of the plastisol.)

Now we can look at the "Z" scheme for transferring electrons from water to NADPH, using photon energy to create a very powerful oxidizing agent (a positively charged intermediate) which can steal the electron from water molecules, while pumping protons as in the mitochondria.

PHOTOSYNTHESIS - The Z Scheme

for the Light Reactions

Let's look a bit at chlorophyll (Figure 18.2, p 532) and its interaction with light.[overheads] Find that there are many chlorophylls/active center. That is, only a small portion of the chlorophyll pool is actually involved in using the light energy (1/300 in Chlorella). What do the rest do? Act together as an antenna (Figure 18.4, p 533) [overhead, L]. The transfers between molecules take <10^-10 sec with an efficiency of >90%. Higher plants also have b-carotenes (p 535) to absorb other light frequencies. Aquatic plants have different dyes, since only blue-green light penetrates to depth.

So what is light energy used for? Look at the "Z" scheme for photosynthetic electron transport. (Figure 18.12, p 542) [overhead] Note the three major complexes are not directly connected. Like the mitochondrial complexes they are connected by carriers which diffuse between them. The cytochrome b₆/cytochrome f complex is analogous to Complex III in mitochondria: same electron path and a (plastoquinone) Q cycle for proton pumping.

The initial removal of electrons from water to give oxygen uses a manganese complex.

Pathway Diagrams

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Last modified 3 December 2001